Obtaining an understanding of subterranean characteristics can be important for oil exploration and other activities that benefit from an enhanced knowledge of the subterranean environment. Various sensors and techniques are used to obtain information related to rock formations, fluid reservoirs, and other features of the subject environment. In some applications, wellbores or other boreholes are drilled and sensors are lowered downhole to help obtain information on surrounding formations.
Various techniques have been employed to obtain gravity measurements that are useful in understanding the features and characteristics of the subterranean region. For example, gravity measurements can be used to determine the densities of the rock and other features surrounding a wellbore. The density data is useful in oil and gas exploration, for example, by helping determine oil and gas distribution throughout a given reservoir region.
As reported by R. L. Mossbauer in Zeit. F. physic 151, 124-143 (1958), it is possible to detect a Doppler shift (or velocity shift) in the frequency of low energy gamma rays emitted by a radioactive isotope using a gamma ray absorbing isotope exhibiting resonance absorption within the range of the shifted frequency. By minimizing the recoil energies of the emitting and absorbing nuclei, such resonance may exhibit a very narrow line width, for example, one part in 1012, thereby obtaining high sensitivity to a correspondingly slight Doppler shift. By way of example, radioactive cobalt-57, having a convenient half-life of 270 days, decays into excited nuclei of iron-57, which in turn emits 120 and 14.4 keV gamma rays in cascade. The emission of the 14.4 keV gamma ray has a half-life of 10−7 seconds. Specially prepared foils enriched with iron-57 isotope have an absorber in close proximity to the cobalt-57 source, and therefore a narrow and very substantial resonance absorption may be observed. There are many types of metals that can be used to observe this so called Mossbauer effect. Here we will use the iron-57 isotope by way of example only.
In a typical demonstration of the Mossbauer effect, either the source or the absorber remains stationary, and the other is moved, for example, by an audio loud-speaker cone, a rotating platform, or a constant velocity drive. To obtain a measure of the absorption characterizing the effect, a scintillation counter is typically used on the face of the absorber film opposite the source so as to be responsive to the gamma rays passing from the source to the absorber film.